1. FIELD OF THE INVENTION
The present invention concerns an exhaust gas purifying catalyst which decomposes, into non-toxic gases, nitrogen oxides discharged from mobile internal combustion engines used, for example, in diesel cars, stationary internal combustion engines used, for example, in cogeneration systems and various industrial furnaces such as boilers, as well as a method of purifying the exhaust gas using such catalyst.
2. DESCRIPTION OF THE RELATED ART
Generally, exhaust gases discharged from automobiles, stationary internal combustion engines and various industrial furnaces contain a great amount of nitrogen oxides represented by NO and NO.sub.2 (NO.sub.x). It is said that such NO.sub.x not only causes photochemical smog but also induces disease in man's respiratory organs.
As a method of decreasing NO.sub.x, an exhaust gas processing technique of a so-called ternary catalyst system is established that eliminates NO.sub.x by reduction in an exhaust gas of low oxygen content, such as that from gasoline cars, by using a reducing agent such as carbon monoxide or hydrocarbon.
On the other hand, in the case of an exhaust gas containing a great amount of oxygen, such as that discharged from large-scaled stationary exhaust emission sources such as boilers, a selective NO.sub.x reduction process for decreasing the amount of NO.sub.x by external addition of ammonia is now under actual operation, which produces some effect.
However, the former method is applicable only to an exhaust gas from a gasoline engine in which the oxygen concentration is extremely low whereas the latter method is difficult to use in small-sized stationary exhaust emission sources or mobile exhaust emission sources from a standpoint of handling because ammonia is used.
In view of the above, various methods have been studied for using hydrogen, carbon monoxide or various hydrocarbons as reducing agents other than ammonia but most of them have the drawback that they are a non-selective catalytic reduction process which can eliminate nitrogen oxides only after oxygen in the exhaust gas has been consumed completely.
Although the following methods have been proposed so far as a novel selective catalytic reduction process capable of overcoming such a drawback (a method of selectively reducing and eliminating nitrogen oxides even under the coexistence of oxygen), none of them can provide quite satisfactory results.
That is, Japanese Patent Laid-Open Hei 2-149317 proposes a method of using (1) a catalyst comprising a hydrogen type mordenite or clinoptilolite or (2) a catalyst comprising a hydrogen type mordenite or clinoptilolite carrying a metal such as Cu, Cr, Mn, Fe and Ni, and bringing an exhaust smoke containing oxygen resulting from combustion of various fuels into contact with the above-mentioned catalyst under the coexistence of an organic compound thereby eliminating nitrogen oxides in the exhaust smoke.
According to this method, a denitrating ratio of 30 to 60% is obtained under the conditions of a reaction temperature of 300 to 600.degree. C. and a gas hourly space velocity (GHSV) of 1200 h.sup.-1, but a denitrating effect under a high GHSV condition, i.e., a condition approximate to that of practical use is not clear. Further, the literatures gives no description of the aging of the catalytic activity and the life of the catalyst is not clear. Further, since the catalyst is evaluated for a pseudo exhaust gas containing no SO.sub.x, the resistance of the catalyst to SO.sub.x is uncertain.
Japanese Patent Laid-Open Hei 1-130735 proposes a method of using a catalyst in which a zeolite ion-exchanged with a transition metal (Cu, Co, Ni, Fe, Mg, Mn or the like) is carried on a refractory support, and capable of purifying nitrogen oxides even in an oxidative atmosphere.
This is a method of purifying nitrogen oxides in an exhaust gas from a gasoline engine at high efficiency even in a lean air/fuel ratio region, in which the oxygen concentration in the exhaust gas is only about 3% at the highest. Accordingly, it is uncertain whether or not nitrogen oxides can be selectively denitrated by reduction also in an exhaust gas such as that from a diesel engine in which the oxygen concentration is from 5 to 10%. Also in the examples, the NO.sub.x reduction tends to be lowered greatly along with an increase in the oxygen concentration.
Japanese Patent Laid-Open Sho 63-283727 proposes a method of using a catalyst in which a metal such as Cu, V, Mn, Fe or Cr is carried on a hydrophobic zeolite with a SiO.sub.2 /Al.sub.2 O.sub.3 ratio of 15 or more and decreasing nitrogen oxides in an oxygen containing exhaust gas from an internal combustion engine under the presence of carbon monoxide and one kind or more of hydrocarbons.
In this method, the denitrating ratio is decreased to as low a value as 4 to 26% in the case of using a zeolite catalyst carrying a metal other than copper. On the other hand, in the case of using a copper-zeolite catalyst, there is the problem that the copper ingredient is readily poisoned by SO.sub.x though a relatively high activity can be obtained. The oxygen concentration in the exhaust gas shown in the examples is 1.6% and it is uncertain whether or not nitrogen oxides can also be reduced selectively for denitration if the oxygen concentration is higher, for example, as in an exhaust gas from a diesel engine.
Japanese Patent Laid-Open sho 63-100919 proposes a method of using a catalyst in which copper is carried on a porous support such as of alumina, silica or zeolite and eliminating nitrogen oxides in an exhaust gas containing oxygen in the presence of a hydrocarbon.
In this method, the denitrating ratio is from 10 to 25% and no high denitrating activity is obtainable. Further, since the catalyst contains copper, there is the problem that the copper ingredient is readily poisoned by SO.sub.x. Further, the oxygen concentration in the exhaust gas shown in the examples is 2.1% and it is uncertain whether or not nitrogen oxides can also be reduced selectively for denitration if the oxygen concentration is higher.